Basement Mold Removal and Musty Smells: Find the Source and Fix It

Basement mold removal works only when you stop the moisture first: seepage through walls or floors, condensation on cold concrete, high humidity, or a past flood that was never fully dried. Then clean hard surfaces such as concrete and block, remove moldy porous materials such as drywall, carpet and insulation, and keep humidity below 60 percent. A musty smell is often the first sign, and the EPA says a moldy odor should be investigated even when you cannot see mold.

What does mold smell like?

People usually describe mold as musty, earthy or like damp cardboard or wet socks. The EPA explains that molds release gases called microbial volatile organic compounds (mVOCs), which are the source of the "moldy odor" often associated with mold growth. It says a moldy odor suggests mold is growing in the building and should be investigated (EPA, What does mold smell like?).

There is no special smell for "black mold" or any one species. A smell tells you there is likely mold and moisture somewhere, not what kind. The EPA notes the health effects of breathing mVOCs are largely unknown, though exposure has been linked to symptoms such as headaches, nasal irritation, dizziness, fatigue and nausea (EPA). If you have symptoms, see a doctor; our mold exposure symptoms guide has more.

Where basement moisture comes from

Basements sit against damp soil and are usually cooler than the rest of the house, so they collect water from several directions.

A common screening trick for damp walls is to tape a square of aluminum foil tightly to the wall and check it after a day or two. Moisture on the room side points to condensation; moisture between the foil and the wall points to water coming through the wall. It is a rough test, not a diagnosis, and both problems can happen at once.

How to remove mold in a basement

The EPA says if the moldy area is less than about 10 square feet, in most cases you can handle the job yourself (EPA). For bigger areas, or after floodwater or sewage, use a professional mold remediation company.

1. Stop the water. Nothing else lasts until this is done.
2. Protect yourself. Wear an N-95 respirator, gloves and goggles (EPA). Keep children and pets out.
3. Dry the space. Run a dehumidifier and fans. The EPA's water damage table for concrete and cinder block lists removing water with an extraction vacuum and drying with dehumidifiers, fans and heaters (EPA Table 1).
4. Remove porous materials that are moldy. The EPA says absorbent materials such as ceiling tiles and carpet may have to be thrown away if they become moldy (EPA). That usually includes finished-basement drywall low on the walls, fiberglass batts, carpet and pad, and cardboard boxes.
5. Scrub hard surfaces. Clean concrete, block, metal and plastic with detergent and water, then dry completely (EPA).
6. If you use bleach, the CDC says use no more than 1 cup per gallon of water, ventilate, and never mix it with ammonia or other cleaners (CDC). Bleach and vinegar together make chlorine gas (Poison Control).
7. HEPA vacuum once everything is dry. The EPA recommends HEPA vacuums for final cleanup after materials are dry and contaminated materials are removed (EPA).
8. Don't paint or seal over mold. Clean and dry first; the EPA says paint over mold is likely to peel (EPA).

Don't run your central HVAC if you suspect the system or ducts in the basement are contaminated (EPA). Our mold in dehumidifiers and HVAC guide covers that situation.

Bleach or vinegar for basement mold?

Neither replaces removal and drying. The EPA's remediation guidance says it is necessary to clean up mold, not just kill it, because dead mold is still allergenic, and it does not recommend bleach as a routine practice (EPA). Physical cleaning and drying matter more than which product you use.

Will a dehumidifier help with mold?

A dehumidifier helps prevent new growth by keeping humidity in range, but it does not remove mold that is already there. ENERGY STAR says the optimum relative humidity in a home is between 30 and 50 percent and that anything above that range may promote mold growth (ENERGY STAR). The EPA recommends keeping indoor humidity below 60 percent, ideally 30 to 50 percent, and checking it with an inexpensive hygrometer (EPA).

Practical tips:

• Size it for the space. ENERGY STAR rates capacity in pints per day and says it is better to oversize than undersize (ENERGY STAR).
• Drain it continuously to a floor drain or sump if you can, so a full bucket does not shut it off.
• Keep it clean. If the dehumidifier itself smells musty, the bucket, filter or coils may be growing mold. Clean it as the manufacturer's manual directs.
• Fix drainage too. ENERGY STAR notes that improving drainage around the foundation can lower basement humidity (ENERGY STAR).

Other musty smells around the house

The same rule applies everywhere: a mold smell means moisture is sitting somewhere.

• Mold smell in the house generally. Check basements, under sinks, around tubs and toilets, behind the washer and in attics. Look for stains, soft spots and warped materials.
• Mold smell from the AC. Often a clogged condensate drain or dirty coil or drain pan. The EPA's guidance mentions keeping HVAC drip pans clean, flowing and unobstructed (EPA guide, PDF).
• Mold smell in the washer. Wet gaskets, detergent drawers and lids left closed hold moisture. Follow the maker's cleaning cycle and leave the door open to dry.
• Mold smell in clothes. Clothes left damp or stored in a humid basement pick up the odor. Wash per the care label and dry completely. Items that stay musty after washing, or that were soaked in floodwater, may not be salvageable.
• Mold smell in a car. Usually a water leak or wet carpet. Dry and clean the interior and find the leak.

How expensive is it to remove mold from a basement?

Published figures differ. Using Angi data, This Old House lists basement mold removal at $500 to $3,000 (This Old House), while Fixr lists $1,500 to $4,000 (Fixr). The main drivers are how much finished material (drywall, carpet, insulation) has to come out, how big the affected area is, and whether the water was clean or from a flood or sewage backup. Waterproofing and rebuild are usually separate. See the full mold remediation cost guide.

When to call a professional

• Mold covers more than about 10 square feet.
• The basement flooded or had a sewage backup. The EPA advises a professional with experience in contaminated-water cleanup (EPA); see sewage cleanup.
• The finished basement has mold behind walls or under flooring.
• Someone in the home has asthma, mold allergy, or a weakened immune system. The CDC says people with asthma or mold allergies may have severe reactions (CDC).

Professionals generally follow standards like the IICRC S520 Standard for Professional Mold Remediation. For recent water, start with flooded basement cleanup or water damage restoration; the EPA says drying within 24 to 48 hours helps prevent mold growth (EPA). The mold remediation overview explains the full process.

Preventing basement mold

• Keep humidity at 30 to 50 percent with a dehumidifier and a hygrometer.
• Keep gutters clear and extend downspouts away from the foundation.
• Slope soil away from the house.
• Insulate cold-water pipes that sweat.
• Vent the dryer outdoors.
• Store items in plastic bins off the floor, not cardboard.
• Avoid carpet on concrete that gets damp; the EPA advises against installing carpet where moisture is persistent, including leaky concrete floors (EPA).
• Test the sump pump before storm season.

Getting help

Flood Rescue Pros is a free service that connects homeowners with independent local water damage and mold restoration companies. We do not do the work ourselves. To talk with a local company about your basement, get help now or find your city on our locations page.

Frequently asked questions

What does house mold smell like?

Most people describe it as musty or earthy, like damp cardboard. The EPA says the odor comes from gases molds release (mVOCs) and that a moldy odor should be investigated.

Will a dehumidifier help with mold?

It helps prevent new growth by keeping humidity below 60 percent, ideally 30 to 50 percent, but it does not remove existing mold. Clean or remove the mold first, then keep humidity down.

Why does my dehumidifier smell like mold?

The bucket, filter or coils may be growing mold, or the unit is pulling in air from a moldy area. Clean it per the manual and look for mold in the space it serves.

How expensive is it to remove mold from a basement?

This Old House, using Angi data, lists $500 to $3,000; Fixr lists $1,500 to $4,000. Removing finished drywall, carpet and insulation raises the cost.

Is bleach or vinegar better to kill black mold?

The EPA focuses on removing mold, not killing it, and does not recommend bleach as a routine step. If you use bleach, keep it to 1 cup per gallon of water and never mix it with vinegar or ammonia.

What should you never clean mold with?

Never mix bleach with ammonia or vinegar. Don't dry sand or dry brush large mold areas without protection, and don't paint over mold.

Key facts

• A musty smell comes from gases molds release (mVOCs); the EPA says a moldy odor should be investigated.
• Basement mold comes from seepage, condensation, humidity, leaks or past flooding; fix the source first.
• Clean concrete and block with detergent and water; moldy drywall, carpet, insulation and cardboard usually go.
• Keep humidity below 60 percent, ideally 30 to 50 percent; a dehumidifier prevents growth but does not remove mold.
• Published basement removal ranges: $500 to $3,000 (This Old House, Angi data) and $1,500 to $4,000 (Fixr).
• Areas over about 10 square feet, floodwater or sewage call for a professional.